Phase transitions and functionality of enzymes and yeasts in dehydrated matrices

摘要

Enzymes and yeast cells were chosen as model probes to investigate functional changes taking place in freeze-dried systems containing different sugars, polymers and salts undergoing phase/state transitions. Differential scanning calorimetry, X-rays diffraction and microscopy provided information on molecular and microscopic changes. Enzyme stability depended on the amorphous structure and non-reducing characteristics of sugars, and residual activity was very low in the super-cooled region when sugars crystallised. The addition of polymers or salts extended the protective effect of sugars to the super-cooled region by delaying crystallisation. Starch provided adequate protection to the enzyme at high water content and low temperatures, where stability was completely lost in other systems. The presence of low-molecular-weight saccharides during drying and storage appeared to be the critical factor protecting cell viability. State diagrams are helpful to analyse the combined effects of temperature, water content and physical state on the stability of biomaterials but there is no direct relationship between overall physical properties and the kinetics of deteriorative changes. Additional information on molecular interactions is relevant to define strategies to preserve the functionality of biomolecules.